Neoplastic diseases, characterized by the proliferation of cells not subject to the normal control of cell growth, are a major cause of death in humans. Clinical experience in cancer chemotherapy has demonstrated that new and more effective drugs are desirable to treat these diseases. Such clinical experience has also demonstrated that drugs which disrupt the microtubule system of the cytoskeleton can be effective in inhibiting the proliferation of neoplastic cells.
The microtubule system of eucaryotic cells is a major component of the cytoskeleton and is in a dynamic state of assembly and disassembly; that is, heterodimers of tubulin are polymerized and form microtubules. These microtubules play a key role in the regulation of cell architecture, metabolism, and division and their dynamic character is essential to their normal function in the cell. For example, with respect to cell division, microtubules are created, that is, polymerized from tubulin, to form the mitotic spindle. When the mitotic spindle's use has been fulfilled, the microtubules contained in it are subsequently depolymerizd. Disrupting either the polymerization or depolymerization of microtubules in the cell can inhibit mitosis, and thereby cell proliferation.
An agent which could prevent a cell from proliferating by inhibiting a cells' microtubule dynamic state would be useful in treating cancer, a disease characterized by cells proliferating at abnormally high rates. Indeed, such agents comprise some of the most effective cancer chemotherapeutic agents in clinical use today.
These anti-mitotic agents or poisons are classified into three groups based on their molecular mechanism of action. The first group, including colchicine and colcemid, inhibit the formation of microtubules by sequestering tubulin. The second group, including vinblastine and vincristine, induce the formation of paracrystalline aggregates of tubulin. These two agent's action preferentially inhibits the proliferation of hyperproliferating cells by disrupting mitotic spindle microtubules. The third group, including taxol, promotes the polymerization of tubulin, thereby disrupting the dyanmic state of microtubule polymerization and depolymerization.
However, an agent's having activity as an antimitotic poison does not lead to the conclusion that this agent would evince such activity in a tumor cell nor in a tumor cell with a drug-resistant phenotype. For example, vinca alkaloids such as vinblastine and vincristine are effective against some neoplastic cells and tumors, yet they lack activity against some drug-resistant tumors and cells. One basis for a neoplastic cell displaying drug resistance (DR) or multiple-drug resistance (MDR) is through the over-expression of P-glycoprotein. Compounds which are poor substrates for transport of P-glycoprotein should be useful in circumventing such a MDR phenotype.
Accordingly, the exhibition of the DR or MDR phenotype by many tumor cells and the clinically proven mode of action of anti-microtubule agents against neoplastic cells necessitates the development of anti-microtubule agents cytotoxic to non-drug resistant neoplastic cells as well as cytotoxic to neoplastic cells with a drug resistant phenotype.